Yeah, not a true "heat treat", in the way we know it, but quenching them, just after casting. Heat treat seems to be the accepted term, a mung casters.
Was typing as you posted.
If you're talking about just quenching....all you need to do is drop from the mold into water.
Clip on wheel weights work pretty well for this.
(Avoid the splash or you may get a visit from the tinsel fairy)
The link I posted above is to actual heat treating.
Some things about metallurgy still make me scratch my head. The fact that it takes the quenched bullets two days to reach maximum hardness, after quenching, is one of them.
I understand that the quench makes the antimony molecules migrate toward the outside of the bullet. Just not sure how the 48 hour period afterward affects things.